A work entitled "telecommunications spatiales" in the telecommunications scientific and technical collection published by Masson, 1982, and in particular vol. I thereof at pp. 92 to 94 and pp. 259 to 261, describes firstly the grouping together of a plurality of antennas which are fed simultaneously from a common transmitter with interposed power dividers and phase shifters, with the characteristics of said group of antennas depending both on the radiation pattern of each antenna and on the way in which power is distributed between them in amplitude and in phase. This property is made use of for obtaining a radiation pattern which cannot be obtained using a single radiating source. Further, if the characteristics of the power dividers and of the phase shifters are modified by electronic means, the radiation pattern can be changed quasi-instantaneously. The simplest way of grouping together radiating sources is to constitute an array in which all of the sources are identical and are offset relative to one another merely in translation. This can give rise, in particular, to arrays which are rectilinear or plane.
The above document also describes the use of antennas having reflectors for generating multiple beams, thereby obtaining a saving in weight and making it possible to provide large radiating areas by using deployable structures. In general, this type of antenna is used when it is desired to generate a plurality of narrow beams. In general, the reflector illuminating system is offset relative to the center of the reflector in order to avoid masking any of the radiating aperture. Any masking of this aperture gives rise to an increase in the level of secondary lobes, and this must be avoided at all costs in this type of application. The main reflector may be a paraboloid, for example. The multiple beams are obtained by placing a set of illuminating sources in the vicinity of the focus, with each source corresponding to one of the beams. Since the sources cannot be located exactly at the focus, illumination is not geometrically perfect and as a result phase aberrations arise which degrade the radiation performance somewhat. The following are observed: the radiation pattern is deformed; there is a loss of gain relative to the gain which could be obtained at the focus; and parasitic secondary lobes arise. The greater the curvature of the reflector and the greater the distance from the focus, the greater the resulting degradations. As a result, reflectors must be made as "flat" as possible, i.e. with a large ratio of focal length to aperture diameter. This gives rise to structures which are large in size, thereby raising problems of accuracy and mechanical strength. In addition, mutual parasitic coupling may arise between the various sources, thereby giving rise to additional secondary lobes.
Given that polarization diversity cannot be used (because of multi-path problems in the 1.6 GHz band (L band)), the only solution is that of frequency reutilization (in particular since bandwidth is rigorously limited for users of satellite telecommunications). The feasibility of such a system thus depends on the possibility of using an antenna with frequency reutilization. Another requirement, from the systems point of view, is that the antenna should be capable of adapting to changes in the location of the request for service. This means that the antenna used should have reconfigurable coverage.
The object of the invention is to satisfy this requirement.